The invention concerns a retractable auxiliary and emergency propulsion device for small craft. Preferably, the propulsion device is characterized by hydraulic transmission of motion.
As is well known, in the construction of boats up to 15 to 20 meters in length, even when the propulsive power required may be supplied by a single motor, it is preferable to use two completely autonomous units in order to provide greater security.
Of course, this solution leads to increased cost, greater weight and higher fuel consumption. In fact, a motor of power P consumes less than two motors of power P/2 for obvious mechanical and thermodynamic reasons.
In effect however, such a system provides only relative security. Whenever a problem occurs which leads to the use of one motor only, conditions are such that continuing navigation is difficult. This is particularly true for small displacement craft with lesser inertia. This is due in particular to the following factors:
(a) the propulsive force does not act along the barycenter of the boat and thus tends to rotate the boat, a tendency which can never be totally overcome with the steering apparatus;
(b) for the above reason and due to the reduced effectiveness of the steering organs in compensating for the rotating tendency, the boat cannot maintain a precise course, a problem which becomes even more severe under adverse weather conditions;
(c) the still operational motor may be run under overload conditions and so must be used at considerably reduced power to prevent this; this is even more indispensable for preventing cavitation of the propeller (with consequent loss of thrust) when it is overloaded.
All of the above factors lead to a very low average velocity.